(a) Field of the Invention
The invention relates to a process for concentrating silicon carbide. More particularly, the invention relates to a process of purifying silicon carbide, by separation of silicon carbide while removing its main impurities, carbon and silicon. This separation is carried out by gravimetry in a dense medium.
(b) Description of Prior Art
Silicon carbide is normally produced by means of pyrometallurgical methods in electrical ovens. The latter usually measure about 12 feet wide by 35 feet long. An oven consist of a wall of about 4 feet high of refractory brick. This wall connects two perpendicular walls which form the two ends of the oven. These two ends are the electrodes. The load of recycled material, such as silica, for example, from Saint-Canut (Indusmin), and of coke is prepared by automatic weighing and mixing to thereafter be sent to the furnace by means of conveyors. When the furnace is half full, loading is stopped. A depression is made in the load from one electrode to the other. The depression is thereafter filled with petroleum coke which will serve as nucleus of resistance. The load is then continued until the furnace is filled (about 10 feet). Each load contains about seventy tons of material, which gives from 6 to 12 tons of acceptable carbide. Electrical power of 2,000 kw is thereafter applied. The reaction proceeds in two stages: EQU SiO+2C.fwdarw.Si+2CO and (1) EQU Si+C.fwdarw.SiC (2)
Heating lasts for about 1.5 days. Cooling lasts about 2 days. At this stage, the load is formed of an unconverted material (firesand), containing a compact cylinder of about three feet diameter connecting the two electrodes. The cylinder, when viewed radially from the surface to the center, comprises a two inch outer layer containing 30 to 50% SiC, a six inch layer of small crystal of carbides containing 80 to 96% SiC, a six inch layer of large carbide crystals containing 97% SiC and an empty space of four inches. This empty space corresponds to the petroleum coke resistance which has been consumed.
Now, in order to obtain concentrates having a high content of silicon carbide, it is necessary first to separate the outer layer from the nucleus of silicon carbide of the interior layers. This grey layer is much less crystallized and contains only 30% to 50% of silicon carbide. Secondly, the portion which is of less purity must be separated from the second layer containing 80% SiC, in order to produce a final product containing 97% and less than 0.45% free carbon.
In order to obtain a concentrate of silicon carbide it is necessary to manually sort out the three layers and only the one or the ones which contain desired or acceptable concentrations are retained.
It is obvious that is would be of interest to be able to rely on an automatic process which would overcome manpower problems. Indeed, presently, the concentration of silicon carbide is carried out by manual sorting in all the manufactures which exist in the world.
To our knowledge, there is no patent nor any other reference relative to a process enabling to concentrate silicon carbide otherwise than by manual steps.